6,420 research outputs found
Pairing in ultracold Fermi gases in the lowest Landau level
We study a rapidly rotating gas of unpolarized spin-1/2 ultracold fermions in
the two-dimensional regime when all atoms reside in the lowest Landau level.
Due to the presence of the spin degree of freedom both s-wave and p-wave
interactions are allowed at ultralow temperatures. We investigate the phase
diagram of this system as a function of the filling factor in the lowest Landau
level and in terms of the ratio between s- and p-wave interaction strengths. We
show that the presence of attractive interactions induces a wide regime of
phase separation with formation of maximally compact droplets that are either
fully polarized or composed of spin-singlets. In the regime with no phase
separation, we give evidence for fractional quantum Hall states. Most notably,
we find two distinct singlet states at the filling nu =2/3 for different
interactions. One of these states is accounted for by the composite fermion
theory while the other one is a paired state for which we identify two
competing descriptions with different topological structure. This paired state
may be an Abelian liquid of composite spin-singlet Bose molecules with Laughlin
correlations. Alternatively, it may be a known non-Abelian paired state,
indicated by good overlaps with the corresponding trial wavefunction. By fine
tuning of the scattering lengths it is possible to create the non-Abelian
critical Haldane-Rezayi state for nu =1/2 and the permanent state of Moore and
Read for nu =1. For purely repulsive interactions, we also find evidence for a
gapped Halperin state at nu=2/5.Comment: 12 pages, 9 figs (best viewed in color), published version with
additional evidence for a non-Abelian spin singlet state at filling nu=2/
Completely dark galaxies: their existence, properties, and strategies for finding them
There are a number of theoretical and observational hints that large numbers
of low-mass galaxies composed entirely of dark matter exist in the field. The
theoretical considerations follow from the prediction of cold dark matter
theory that there exist many low-mass galaxies for every massive one. The
observational considerations follow from the observed paucity of these low-mass
galaxies in the field but not in dense clusters of galaxies; this suggests that
the lack of small galaxies in the field is due to the inhibition of star
formation in the galaxies as opposed to the fact that their small dark matter
halos do not exist. In this work we outline the likely properties of low-mass
dark galaxies, and describe observational strategies for finding them, and
where in the sky to search. The results are presented as a function of the
global properties of dark matter, in particular the presence or absence of a
substantial baryonic dark matter component. If the dark matter is purely cold
and has a Navarro, Frenk and White density profile, directly detecting dark
galaxies will only be feasible with present technology if the galaxy has a
maximum velocity dispersion in excess of 70 km/s, in which case the dark
galaxies could strongly lens background objects. This is much higher than the
maximum velocity dispersions in most dwarf galaxies. If the dark matter in
galaxy halos has a baryonic component close to the cosmic ratio, the
possibility of directly detecting dark galaxies is much more realistic; the
optimal method of detection will depend on the nature of the dark matter. A
number of more indirect methods are also discussed.Comment: 12 pages, 4 figures, MNRAS in pres
Particle Entanglement Spectra for Quantum Hall states on Lattices
We use particle entanglement spectra to characterize bosonic quantum Hall
states on lattices, motivated by recent studies of bosonic atoms on optical
lattices. Unlike for the related problem of fractional Chern insulators, very
good trial wavefunctions are known for fractional quantum Hall states on
lattices. We focus on the entanglement spectra for the Laughlin state at
for the non-Abelian Moore-Read state at . We undertake a
comparative study of these trial states to the corresponding groundstates of
repulsive two-body or three-body contact interactions on the lattice. The
magnitude of the entanglement gap is studied as a function of the interaction
strength on the lattice, giving insights into the nature of Landau-level
mixing. In addition, we compare the performance of the entanglement gap and
overlaps with trial wavefunctions as possible indicators for the topological
order in the system. We discuss how the entanglement spectra allow to detect
competing phases such as a Bose-Einstein condensate.Comment: 12 pages, 9 figure
Massive, Absorption-selected Galaxies at Intermediate Redshifts
The nature of absorption-selected galaxies and their connection to the
general galaxy population have been open issues for more than three decades,
with little information available on their gas properties. Here we show, using
detections of carbon monoxide (CO) emission with the Atacama Large
Millimeter/submillimeter Array (ALMA), that five of seven high-metallicity,
absorption-selected galaxies at intermediate redshifts, ,
have large molecular gas masses, and high molecular gas fractions (. Their modest star
formation rates (SFRs), yr, then
imply long gas depletion timescales, Gyr. The
high-metallicity absorption-selected galaxies at appear
distinct from populations of star-forming galaxies at both ,
during the peak of star formation activity in the Universe, and lower
redshifts, . Their relatively low SFRs, despite the large
molecular gas reservoirs, may indicate a transition in the nature of star
formation at intermediate redshifts, .Comment: 8 pages, 3 figures; accepted for publication in Astrophysical Journal
Letters. Minor changes to match the version in press in ApJ
First Results from the Large Area Lyman Alpha Survey
We report on a new survey for z=4.5 Lyman alpha sources, the Large Area Lyman
Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume
and sensitivity by using narrow-band filters on the new 8192x8192 pixel CCD
Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National
Observatory.
Well-detected sources with flux and equivalent width matching known high
redshift Lyman alpha galaxies (i.e., observed equivalent width above 80
Angstroms and line+continuum flux between 2.6e-17 and 5.2e-17 erg/cm^2/sec in
an 80 Angstrom filter) have an observed surface density corresponding to 11000
+- 700 per square degree per unit redshift at z=4.5. Spatial variation in this
surface density is apparent on comparison between counts in 6561 and 6730
Angstrom filters.
Early spectroscopic followup results from the Keck telescope included three
sources meeting our criteria for good Lyman alpha candidates. Of these, one is
confirmed as a z=4.52 source, while another remains consistent with either
z=4.55 or z=0.81. We infer that 30 to 50% of our good candidates are bona fide
Lyman alpha emitters, implying a net density of about 4000 Lyman alpha galaxies
per square degree per unit redshift.Comment: 10 pages, 2 figures (3 .ps files), uses AASTeX 4. Submitted to The
Astrophysical Journal Letter
The Very Highly Ionized Broad Absorption Line System of the QSO SBS1542+541
We have analyzed the broad absorption line system of the bright (V=16.5)
high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS along
with optical data from the MMT and the Steward Observatory 2.3m telescope.
These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms,
corresponding to 340 to 2480 Angstroms in the QSO rest frame. This object
therefore offers a rare opportunity to study broad absorption lines in the
rest-frame extreme UV. We find that the absorption system is dominated by very
high-ionization species, including O VI, NeVIII, and SiXII. We also identify
apparently saturated broad Lyman-series lines of order Ly-gamma and higher.
There is strong evidence for partial occultation of the QSO emission source,
particularly from the higher-order Lyman lines which indicate a covered
fraction less than 0.2. Overall, the data suggest a correlation between a
larger covered fraction and a higher state of ionization. We suggest that the
different covered fractions can be explained by either a special line of sight
through a disk-like geometry or by the existence of density fluctuations of a
factor >2 in the BAL gas. Our photoionization models of the system indicate a
large column density and high ionization state similar to that found in X-ray
``warm absorbers''.Comment: 31 pages, 13 figures, to be published in Ap
Multiphase Plasma in Sub-Damped Lyman Alpha Systems: A Hidden Metal Reservoir
We present a VLT/UVES spectrum of a proximate sub-damped Lyman-alpha
(sub-DLA) system at z=2.65618 toward the quasar Q0331-4505
(z_qso=2.6785+/-0.0030). Absorption lines of O I, Si II, Si III, Si IV, C II, C
III, C IV, Fe II, Al II, and O VI are seen in the sub-DLA, which has a neutral
hydrogen column density log N(H I)=19.82+/-0.05. The absorber is at a velocity
of 1820+/-250 km/s from the quasar; however, its low metallicity
[O/H]=-1.64+/-0.07, lack of partial coverage, lack of temporal variations
between observations taken in 2003 and 2006, and non-detection of N V imply the
absorber is not a genuine intrinsic system. By measuring the O VI column
density and assuming equal metallicities in the neutral and ionized gas, we
determine the column density of hot ionized hydrogen in this sub-DLA, and in
two other sub-DLAs with O VI drawn from the literature. Coupling this with
determinations of the typical amount of warm ionized hydrogen in sub-DLAs, we
confirm that sub-DLAs are a more important metal reservoir than DLAs, in total
comprising at least 6-22% of the metal budget at z~2.5.Comment: 5 pages, 3 color figures, accepted for publication in ApJ
Modelling elliptical galaxies: phase-space constraints on two-component (gamma1,gamma2) models
In the context of the study of the properties of the mutual mass distribution
of the bright and dark matter in elliptical galaxies, present a family of
two-component, spherical, self-consistent galaxy models, where one density
distribution follows a gamma_1 profile, and the other a gamma_2 profile
[(gamma_1,gamma_2) models], with different total masses and ``core'' radii. A
variable amount of Osipkov-Merritt (radial) orbital anisotropy is allowed in
both components. For these models, I derive analytically the necessary and
sufficient conditions that the model parameters must satisfy in order to
correspond to a physical system. Moreover, the possibility of adding a black
hole at the center of radially anisotropic gamma models is discussed,
determining analytically a lower limit of the anisotropy radius as a function
of gamma. The analytical phase-space distribution function for (1,0) models is
presented, together with the solution of the Jeans equations and the quantities
entering the scalar virial theorem. It is proved that a globally isotropic
gamma=1 component is consistent for any mass and core radius of the
superimposed gamma=0 model; on the contrary, only a maximum value of the core
radius is allowed for the gamma=0 model when a gamma=1 density distribution is
added. The combined effects of mass concentration and orbital anisotropy are
investigated, and an interesting behavior of the distribution function of the
anisotropic gamma=0 component is found: there exists a region in the parameter
space where a sufficient amount of anisotropy results in a consistent model,
while the structurally identical but isotropic model would be inconsistent.Comment: 29 pages, LaTex, plus 5 .eps figures and macro aaspp4.sty - accepted
by ApJ, main journa
Comparison of 7T 16-channel Dual-row Transmit Arrays
We evaluated and compared the performance of an inductively decoupled and overlapped dual-row transmit arrays for MRI at 7T. For the evaluated designs, the coupling between adjacent elements in the same row was higher for the overlapped compared to the non-overlapped configuration. However the transmit efficiencies for the circular polarization mode of both coils were similar. For comparisons of array transmit performance, consideration of array-internal losses as well as reflected and radiated power is very important, because their sum can be as high as 55% of the total transmit power
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